1. Field of the Invention
This invention relates to magnetic recording and in particular to perpendicular digital recording in which information is recorded in the form of digital bits.
The demand for increasing performance and capacity of moving media magnetic data storage systems for use with computers has led to the analysis of the limitations of conventional recording technology. In conventional longitudinal recording, recording is accomplished by magnetizing the magnetic recording medium (e.g., iron oxide) in a direction parallel to the surface of the carrier of the medium. Generally, a ring type recording head is employed in which recording is accomplished by passing the medium across the pole gap. The limitations of this technology relate principally to the problems of increasing media demagnetization with increasing recording (i.e., bit) density and the very low energy efficiency of conventional ring type heads. While significant progress has been made in recent years in the development of ultra thin recording media and thin film heads, the above problems remain as fundamental limitations inherent in the conventional longitudinal recording process.
In perpendicular recording, the recording medium is magnetized in a direction perpendicular to the plane of the medium. Essentially all of the flux generated in the poles of the recording head passes through the medium, resulting in a very high energy efficiency. Despite its great potential, a number of problems have inhibited the development of perpendicular recording technology. Notable among these are the lack of media suitable for recording in the perpendicular mode and the lack of a practical head design, particularly a multihead array.
2. Description of the Prior Art
In the initial development of perpendicular recording technology, a head was utilized which resembled a conventional ring type longitudinal recording head, i.e., it included a pair of recording poles having a gap therebetween. Such a head is shown in U.S. Pat. No. 3,454,727 to Siera et al. In order to record in a perpendicular fashion, the recording medium is moved through the gap between the poles. Such a requirement presents substantial problems in terms of transport and medium design. Prior art attempts to overcome this problem have involved the development of new head designs operating in conjunction with a modified recording medium. One form of modified recording medium includes a magnetizable layer which overlies a high permeability substrate. One form of modified head includes a narrow recording pole and a long flux closing pole. The combination of the modified magnetic head and recording medium forms a magnetic circuit, with the high permeability substrate providing a low reluctance return path from one pole of the head to the other. In order to record, a strong magnetic field is applied to the magnetizable layer of the medium in the area underlying the recording pole. The flux then passes through the substrate and back to the recording pole through the long flux closing pole, thereby forming a closed magnetic circuit in which the magnetic flux in the medium directly under the poles of the head is oriented perpendicular to the plane of the recording medium. The cross-sectional area of the long flux closing pole is much larger than that of the narrow recording pole to insure that the flux density at the flux closing pole is insufficient to magnetize the medium. Recording systems of this type are disclosed in U.S. Pat. Nos. 2,840,440 to McLaughlin et al. and 4,138,702 to Magnenet, and in Japanese Pat. Nos. 52-78403 to Iwasawa and 54-59108 to Nakagawa. Other prior art relating to perpendicular recording that should be considered to fully appreciate the subject invention are the following: S. Iwasaki, Y. Nakamura and K. Ouchi, Perpendicular Magnetic Recording with a Composite Anisotropy Film, IEEE Transactions on Magnetics, Vol. 15 No. 6, November 1979; Robert I. Potter and Irene A. Beardsley, Self-Consistent Computer Calculations for Perpendicular Magnetic Recording, IEEE Transactions on Magnetics, Vol. 16, No. 5, September, 1980; U.S. Pat. No. 4,251,842 issued Feb. 17, 1981 to S. Iwasaki et al; Japanese Patent Applications Nos. 51-51574 and 51-106506, respectively laid open to the public on Nov. 11, 1977 under unexamined publication number 134,706/77 and Mar. 25, 1978 under unexamined publication number 32,009/786 and U.S. Pat. No. 4,253,127 issued Feb. 24, 1981 to T. Kodama and T. Yanagida.
In addition to the development of a suitable unitary magnetic head, perpendicular recording technology is confronted in common with longitudinal technology, with the difficulty of fabricating high track density multihead arrays. In order to achieve the highest possible recording density, it is desirable to have as many tracks as possible for a particular width of recording medium. The reduction of head to head spacing in such multihead arrays has been limited by the need to allow adequate space between adjacent poles for the fabrication of read/write windings and their conductors. Both the McLaughlin et al. and Magnenet patents disclose multitrack heads which utilize a separate read/write coil for each head. In Magnenet, track density is doubled by providing two arrays of heads which are offset with respect to each other by a distance equal to one half of the spacing between individual heads. Each array of heads records on alternately spaced tracks on the recording medium.
In addition to physical head design, the mode of operation of a head is important in achieving maximum density. As recording density increases, it becomes more difficult to read the recorded information. Conventional magnetic read heads sense the time rate of change of magnetic flux produced by the motion of the medium past the pole tips of the head. Increasing the relative speed of the medium past the pole tips increases the performance, since the induced flux rate of change is increased.
Various head designs have been developed in which the head is responsive to the intensity of the induced flux rather than the time rate of change of flux. Such flux responsive heads are disclosed in U.S. Pat. Nos. 3,375,332 to Geyder, 3,242,269 to Pettengill, 3,444,331 and 3,444,332 to Brown, Jr., 3,696,218 to Eumura, 4,123,790, 4,136,371, 4,136,370 and 4,182,987 to Moeller, 4,120,011 to Kolb, Jr., 4,137,554 to McClure and 4,164,770 to Jeffers. A common type of flux sensitive head is referred to as a flux gate head and operates by periodically saturating a portion of the high permeability core of a magnetic head, thereby causing the reluctance of the magnetic circuit to increase rapidly to a high value and giving rise to a sudden decrease of flux in the magnetic circuit to a very low level. The flux gate may be activated once each bit period (at the time of maximum flux intensity), thereby producing a rapid change of flux in the magnetic circuit and generating an emf in the sense winding whose amplitude and polarity correspond to the intensity and polarity of the recorded bit. Alternatively, the flux gate may be activated several or more times during each bit period, thereby generating an AC signal whose amplitude varies in accordance with the flux intensity produced by the medium and whose phase (relative to the flux gate current) corresponds to the polarity of residual flux in the medium. The motion of the medium plays no essential role in generating the read signal in either case.
Flux sensitive magnetic heads for use in perpendicular recording have been disclosed in U.S. Pat. Nos. 3,454,727 to Siera et al. and 3,651,502 to Flora. The Siera et al. patent discloses a plural track flux gated head in which flux gate excitation or saturation current is applied to each pole of the head (i.e., every pole is saturated simultaneously). The head of the Flora patent does not utilize a flux gate arrangement. Rather, a bias signal is applied to the head and sensed at the output of the head. The design of the head is such that the magnitude of the output signal will be reduced if a magnetic field is present in the portion of the recording medium being sensed since the magnetic field of the recording medium cancels magnetic flux caused by the application of the bias signal to the head.